This invention relates to refolding fan-folded webs into a stack and, in particular, to an arrangement for sensing the height of stack of fan-fold paper being refolded and controlling the elevation of the stack height to facilitate proper folding.
In data processing and communication applications, high speed printers are used to print the rapidly generated output of data processing machines. The medium on which printing generally is performed is "fan-folded", that is, it comprises an elongated web having transversely extending folds longitudinally spaced with alternate folds pointing in opposite directions. The web is taken into the printer from a stack within which it is tightly folded at the spaced folds. In the course of printing the web is unfolded and a need exists to refold it along its folds in a zigzag manner and into a stack. The web may be a single sheet of paper or consist of several sheets of paper interspersed with carbons. It is not uncommon to print up to several sheets with carbons.
It should be noted that the output of a high speed printer is not necessarily continuous and constant. The printer will generate printout faster when printing short lines than long ones. When slewing (feeding paper without printing), the paper moves through the printer at a extremely high speed. Further with some types of printers, paper is fed from the printer discretely and only after a line has been printed. No paper movement occurs during printing, that is the printer's paper output is not continuous.
Machines have been designed to stack the fan-folded printout from high speed printers. Oftentimes, a pair of paper feed tractors engage edge perforations in the paper and are used to feed the paper from its incoming stack through the printing mechanism and then to the outgoing stack being formed on a platform. The paper exiting from the printing mechanism is directed above the platform and caused to fall controllably into a stack on the platform. A particularly useful stacker is described in copending application of John R. Bittner, Harry R. Berrey and Ralph S. Billings entitled "Continuous Forms Refolder For High Speed Printers", U.S. Ser. No. 29,390 filed concurrently with this application on April 12, 1979 and assigned to a common assignee. This stacker employs an elevator which receives the paper from the printer and has provisions for driving said received paper longitudinally toward the platform. The elevator is designed to be automatically movable relative to the platform during the stacking operation so that a desired range of space is maintained between the elevator and the top of the stack of paper being folded on the platform. The maintaining of this proper spatial relationship results in an improved refolding of the paper on the platform. Reference can be made to this copending application for details of the improved refolding action and how it is achieved.
A need exists for an arrangement which can control such a range of spacing automatically and recurrently. A common approach is to employ an array of radiant energy sensors, such as photoelectric sensors, to sense the top of the stack of paper. The sensing operation is made difficult because of paper fluffing which occurs at the folded ends of the forms and does not represent the true height of the stack. Ambient light, dark leakage current and momentary light interruptions can result in a misreading of stack height. These and other problems to be described have made it difficult to provide a reliable, low cost, inexpensive device for sensing the height of a stack and controlling desirable spacing of the elements in a continuous forms refolder for a high speed printer.